There is volumes of research and studies on animal development. I will spare you the bulk of it, as it’s the type of thing I like to read as a “dog nerd,” but most people would prefer the condensed version, along with some tips.
If you obtained your puppy from a reputable breeder, the breeder would have interacted with the puppies from an early age, day 2 of life, doing early neurological stimulation, also called Bio Sensor Stimulation. Involves handling the pup and putting it through short periods of stress. Things like applying a cool rag for a few seconds, touching the foot, holding the pup on its back. It produces more confident, well adjusted dogs who are not easily stressed and are resilient. You would find more detail on this in the book “Breeding Better Dogs” by Dr. Carmen Battaglia, PHD. The breeder may have also started socializing the puppies to new people. I hope if you purchased your puppy it was from a breeder of quality, healthy, mentally sound dogs. But not everyone obtains their dog from a breeder at all, and so we can’t always say what our dogs life was like before.
There are some critical periods in your puppy’s growth. From 7 weeks onward to 16 wks, there is rapid growth. This is the time most people take their puppies home, at about 8 weeks. Puppies also go through fear periods, so you must avoid frightening your dog at these times, or it may have lasting effects. For example, you would never throw your puppy in the water to teach him to swim, you would introduce it slowly. Don’t try to “soothe” a dogs fears with kind words or petting, because this is taken as reinforcement. Think of all the things that could potentially frighten an adult dog… now introduce all these things to your puppy, from a safe distance and coming closer. Vacuum cleaners, the L train, loud noises, riding in the car, getting nails clipped, getting ears cleaned, children, hair dryers. I’ve met several adult dogs who were startled by a garbage bag being opened! Most timidity is preventable.
Environmental things. Have your puppy run over new surfaces. I used to feed my puppy by putting her food inside a tube she had to climb into. This tube may have been viewed as “scary” to some dogs, but this puppy was not scared at all. Use treats and food as rewards and a positive association. Also practice holding the food bowl, putting your hand in it, and some hand-feeding so the pup is comfortable with people touching his food. You don’t need to keep trying to take it away though, you don’t want to create an issue. That is a myth. Another myth is that you have to stare your puppy in the eyes or pin him on his back. Do not allow your puppy to bite you or your hands/clothes, avoid this or use gentle discipline. It’s probably not a good idea to allow your pup to sleep with you in bed. He should have a place of his own, such as a dog crate in your bedroom.
Besides environmental, you have social. Have as many people as possible handle the puppy, walk them around, give them treats, feel their paws and rub their gums. Bring your puppy to all family events. Join a puppy class! Allow your puppy to socialize only with dogs who will not bully or dominant them, preferably calm older dogs or other friendly puppies. Some people make the mistake of thinking the dog(s) they already have at home are enough to socialize, but it’s not. The puppy must meet new dogs. This is why a puppy class is good. But be careful- If your puppy is hurt or bullied by another dog during this critical period in life, you may have lasting effects that will cause problems later.
If you have an adult dog, it’s not too late to socialize, although it won’t be as easy as socializing a puppy you can do many of the same things with an adult dog. Make every experience a positive one for your dog, and keep your dog safe. A dog who has leadership and a foundation of obedience will be a lot easier to socialize. If you have a puppy, now is the time to start with leadership and teaching them a hierarchy within the family unit.